Fix backend matmul array-like inputs

[FlorianPfaff]

Summary

• Convert shared NumPy backend matmul operands before checking dimensionality.
• Preserve the existing explicit rejection of 1-D vector operands.
• Add regression coverage for list-like matrix operands and NumPy-style out.

Rationale

The public pyrecest.backend.matmul helper advertises backend operations through a NumPy-style facade. The shared NumPy implementation checked .ndim before coercing inputs, causing list-like matrix operands to fail with an attribute error instead of working like matrix inputs.

Validation

• Fresh branch created from current main.
• Added focused regression tests in tests/test_backend_matmul_contract.py.
• Full local pytest was not run because this execution container cannot resolve github.com; repository CI should run the focused tests and full suite.

[github-actions]

✅MegaLinter analysis: Success

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✅ PYTHON	black	767	0	0	0	35.92s
✅ PYTHON	isort	767	0	0	0	2.62s
✅ REPOSITORY	checkov	yes		no	no	44.07s
✅ REPOSITORY	gitleaks	yes		no	no	14.7s
✅ REPOSITORY	git_diff	yes		no	no	0.09s
✅ REPOSITORY	secretlint	yes		no	no	22.49s
✅ REPOSITORY	syft	yes		no	no	4.69s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.97s
✅ REPOSITORY	trufflehog	yes		no	no	22.03s
✅ YAML	prettier	11	0	0	0	0.73s
✅ YAML	v8r	11		0	0	10.59s
✅ YAML	yamllint	11		0	0	0.45s

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